Influence of Night Temperature on Disease Development in Fusarium Wilt of Chrysanthemum

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Abstract
Constant high temperatures are known to favor development of fusarium wilt of florists'' chrysanthemum caused by Fusarium oxysporum f. sp. chrysanthemi. However, temperatures in greenhouses and the field rarely remain high for long periods of time. Rooted cuttings of Chrysanthemum morifolium ''Royal Trophy,'' "Mandalay,'' and ''Torch'' were grown for 2 wk and root-inoculated with F. o. f. sp. chrysanthemi. Plants were exposed in regimes with a day temperature of 35 C and night temperatures of 13, 18, 24, 29, or 35 C. Symptoms were rated daily on a scale of 0 (no symptoms) to 5 (dead) for 28 days after inoculation. Average total ratings (ATR) were obtained by summing the daily ratings and dividing by the number of inoculated plants. For cultivars Royal Trophy and Mandalay, ATR were greatest at a night temperature of 29 C. For Torch, ATR did not vary significantly with night temperature. Symptoms in Torch were not apparent until night temperatures were 24 C or greater, whereas other cultivars exhibited symptoms at all temperatures. Symptomless plants of all cutivars were colonized frequently. Effects of diurnal temperatures on mycelial growth and population of the pathogen on agar media were not correlated with disease development.

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